Hey, I'm trying to repair an amplifier that I destroyed. I was replacing the differential amplifier transistor pairs (they were causing high DC offset), when I saw a spark somewhere on the board (I forgot to discharge the main filter capacitors) and now I have 29VDC offset on one channel (supply voltage is +/-33V). It still outputs sound, but the sound is static only (same sound as an old tv with no signal). The junk speaker I had connected started smelling like burnt voice coil (still works though), which is how I detected the high offset. The other channel works fine. I've tried putting the old transistors back but no difference.

When I unplug the amp, the + supply quickly goes down to about 7V, whereas the - supply goes down to -12V, but at a much slower rate.

I've tried many things, but am totally stuck. I cannot understand what is causing this. Got any ideas for me?

To get that much dc at the output and to have that rail collapse quickly after power down you must have a lot of current flowing from the +ve rail to ground. By measuring the voltage drop across resistors in the circuit you can discover where this current is flowing - and where it should not be flowing through a blown part. I would suspect the output (and driver) transistor on the +ve side as the first place to look.

__________________"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

I would look at the front end 2sa798. Yank it out and check for shorts.

That is the one I replaced originally, the producedure that killed the amp (by something shorting), with two KSA992 transistors (wired in correctly). Currently I have put the old 2SA798 back, as a test, but the problem remains the same.

Update on the problem: I have now pulled every singly transistor on the blown side, and none of them are faulty.

And, with all the transistors physically missing - the B+ is still dropping rapidly after power down (whereas the B- goes much slower).

I will try to check some resistors next.

Also note that the phono pre amp is also using the B+ to create its own supply of 29 volts. EDIT: Disconnected. No difference. Tuner's been disconnected as well.

EDIT again: The transistor on the power supply board (Q501) has the intended voltages on the schematic. B = 14.2V C = 28.9V and E = 13.6V. The voltages I get here are: B = 12.1 C = 32.6 E = 11.6. I think this is a bit odd, because B and E are lower than intended, whereas C is higher. The emitter voltage goes to pin 14 on the main amp board (as seen to the left on the schematic). I think it might be important.

Already done that too. They consist of two big, dual diodes. One with + - + polarity, and the other with - + -. They appear fine. 0.6V in all the right directions (and no conductivity in the others). I did physically remove them before testing.

Have you checked the output transistors? You can start to diode-measure them. NPN: Plus (red wire on your multimeter) at the base and minus (black wire) at the emitter and later the collector. There you should get something and since you have a working channel you can compare.